Aromatic hydrocarbons, particularly benzene, toluene, ethylbenzene and xylenes, are important commodity chemicals in the petrochemical industry. Currently, aromatics are most frequently produced from petroleum-based feedstocks by a variety of processes, including catalytic reforming and catalytic cracking. However, as the world supplies of petroleum feedstocks decrease, there is a growing need to find alternative sources of aromatic hydrocarbons.
One possible alternative source of aromatic hydrocarbons is methane, which is the major constituent of natural gas and biogas. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of alkanes contained in natural gas directly to higher hydrocarbons, such as aromatics, is an attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome.
A large majority of the processes currently proposed for converting methane to liquid hydrocarbons involve initial conversion of the methane to synthesis gas, a blend of H2 and CO. However, production of synthesis gas is capital and energy intensive and hence routes that do not require synthesis gas generation are preferred.
A number of alternative processes have been proposed for directly converting methane to higher hydrocarbons. One such process involves catalytic oxidative coupling of methane to olefins followed by the catalytic conversion of the olefins to liquid hydrocarbons, including aromatic hydrocarbons. For example, U.S. Pat. No. 5,336,825 discloses a two-step process for the oxidative conversion of methane to gasoline range hydrocarbons comprising aromatic hydrocarbons. In the first step, methane is converted to ethylene and minor amounts of C3 and C4 olefins in the presence of free oxygen using a rare earth metal promoted alkaline earth metal oxide catalyst at a temperature between 500° C. and 1000° C. The ethylene and higher olefins formed in the first step are then converted to gasoline range liquid hydrocarbons over an acidic solid catalyst containing a high silica pentasil zeolite.
However, oxidative coupling methods suffer from the problems that they involve highly exothermic and potentially hazardous methane combustion reactions and they generate large quantities of environmentally sensitive carbon oxides.
A potentially attractive route for upgrading methane directly into higher hydrocarbons, particularly ethylene, benzene and naphthalene, is dehydroaromatization or reductive coupling. This process typically involves contacting the methane with a catalyst comprising a metal, such as rhenium, tungsten or molybdenum, supported on a zeolite, such as ZSM-5, at high temperature, such as 600° C. to 1000° C.
For example, U.S. Pat. No. 4,727,206 discloses a process for producing liquids rich in aromatic hydrocarbons by contacting methane at a temperature between 600° C. and 800° C. in the absence of oxygen with a catalyst composition comprising an aluminosilicate having a silica to alumina molar ratio of at least 5:1, said aluminosilicate being loaded with (i) gallium or a compound thereof and (ii) a metal or a compound thereof from Group VIIB of the Periodic Table.
In addition, U.S. Pat. No. 5,026,937 discloses a process for the aromatization of methane which comprises the steps of passing a feed stream, which comprises over 0.5 mole % hydrogen and 50 mole % methane, into a reaction zone having at least one bed of solid catalyst comprising ZSM-5, gallium and phosphorus-containing alumina at conversion conditions which include a temperature of 550° C. to 750° C., a pressure less than 10 atmospheres absolute (1000 kPaa) and a gas hourly space velocity of 400 to 7,500 hr−1.
Moreover, U.S. Pat. Nos. 6,239,057 and 6,426,442 disclose a process for producing higher carbon number hydrocarbons, e.g., benzene, from low carbon number hydrocarbons, such as methane, by contacting the latter with a catalyst comprising a porous support, such as silica, alumina, zeolite ZSM-5, FSM-16 and mixtures thereof, which has dispersed thereon rhenium and a promoter metal such as iron, cobalt, vanadium, manganese, molybdenum, tungsten or a mixture thereof. After impregnation of the support with the rhenium and promoter metal, the catalyst is activated by treatment with hydrogen and/or methane at a temperature of about 100° C. to about 800° C. for a time of about 0.5 hr. to about 100 hr. The addition of CO or CO2 to the methane feed is said to increase the yield of benzene and the stability of the catalyst.
Further in our International Patent Publication No. WO 2006/068814, published Jun. 29, 2006, we have described a process for converting methane to higher hydrocarbons including aromatic hydrocarbons, the process comprising contacting a feed containing methane with a dehydrocyclization catalyst, conveniently molybdenum, tungsten and/or rhenium or a compound thereof on ZSM-5 or an aluminum oxide, under conditions effective to convert said methane to aromatic hydrocarbons and produce a first effluent stream comprising aromatic hydrocarbons and hydrogen, wherein said first effluent stream comprises at least 5 wt % more aromatic rings than said feed; and reacting at least part of the hydrogen from said first effluent stream with an oxygen-containing species to produce a second effluent stream having a reduced hydrogen content compared with said first effluent stream.
However, the successful application of reductive coupling to produce aromatics on a commercial scale requires the solution of a number of serious technical challenges. For example, the reductive coupling process is both endothermic and thermodynamically limited. Thus the cooling effect caused by the reaction lowers the reaction temperature sufficiently to greatly reduce the reaction rate and total thermodynamic conversion if significant make-up heat is not provided to the process.
In addition, the process tends to produce carbon and other non-volatile materials, collectively referred to as “coke”, that accumulate on the catalyst resulting in reduced activity and potentially undesirable selectivity shifts, as well as loss of valuable feedstock. Although the coke can be removed from the catalyst by oxidative or reductive regeneration, this leads to lost production time as well as potential damage to the catalyst. There is therefore interest in developing dehydrocyclization catalysts that exhibit reduced coke selectivity without loss in selectivity to the desired aromatic products.
One particularly active catalyst for the dehydroaromatization of methane is molybdenum on an aluminosilicate zeolite, such as ZSM-5, particularly where the molybdenum is highly dispersed and anchored at Bronsted acid sites in the pores of the zeolite. Such a catalyst normally also includes a refractory binder, typically alumina, to provide the catalyst particles with the requisite size, density and hardness for use in the dehydroaromatization process. However, it has now been found that the alumina binder can interact with molybdenum during catalyst manufacture and use to produce bulk (that is not anchored to zeolite Bronsted acid sites) aluminum molybdate that can have a detrimental impact on the performance of the catalyst. Thus, bulk aluminum molybdate is found to be highly active for methane conversion with essentially 100% conversion to coke. The present invention therefore seeks to provide a catalyst and process for conversion of methane to aromatics in which the alumina binder is replaced by other refractory materials which provide the catalyst with desired physical properties without impairing its catalytic performance and without providing reactive non-framework aluminum.
International Patent Publication No. WO 1998/051409, published Nov. 19, 1998, discloses a catalyst for converting C2 to C16 hydrocarbons to olefins and aromatics with a low rate of coke formation, the catalyst comprising an acid treated zeolite, such as ZSM-5, impregnated with zinc and at least one other metal selected from the group consisting of Group 4B, Group 6B, Group 3A, Group 4A and Group 5A of the Periodic Table of Elements. The catalyst can include a binder selected from alumina, silica, alumina-silica, aluminum phosphate, clays and mixtures thereof.
U.S. Published Patent Application No. 2004/266608, published Dec. 30, 2004, discloses a molecular sieve-containing catalyst for cracking hydrocarbons, comprising molecular sieve, refractory inorganic oxide, clay and a metal component, wherein the amount of said molecular sieve is from 1 to 90% by weight, the refractory inorganic oxide is from 2 to 80% by weight, the clay is from 2 to 80% by weight, and the metal component is from 0.1 to 30% by weight, calculated as the oxide of said metal having its maximum valence state, based on the total amount of the catalyst, wherein said metal component exists essentially in a reduction state and is one or more metals selected from the group consisting of metals of Group III A (other than aluminum), metals of Group IVA, VA, IB, IIB, VB, VIB and VIIB, and non-noble metals of Group VIII of the Periodic Table. The metal component is conveniently selected from gallium, tin, copper, silver, zinc, vanadium, molybdenum, manganese, iron, and/or cobalt, and refractory inorganic oxide is conveniently selected from alumina, silica, amorphous silica-alumina, zirconia, titanium oxide, boron oxide, and oxides of alkaline earth metal.